On one of Jose Rodriguez's vids on Pro 1000, I remember him saying that it is better for the life of the print head if you set the printer on Quiet mode (in printer settings). From what he said it followed that the printer slows down a little & doesn't overheat. I have that setting as default on mine, it doesn't slow it down a great deal but my print head is going well! I have no actual data to offer but it cannot do any harm as far as I know & it has had no ill effect on usage or print quality. But I don't print 6 or 8 hours continuously. I think as you're printing relatively small prints at 5x7 there will be a natural pause between prints which should also help. Quiet setting may also ward off potential pauses in printing due to overheating.
Canon should be able to advise too on best practice for continuous printing. All best with it.
